Metal lines over insulators and ground planes, metal lines buried in close proximity to dielectric insulators and interconnection lines on interposers and printed circuit boards are commonly used in conventional interconnects. Integrated circuit speed performance is determined not by transistor switching speeds, but by interconnect delays. As the size of metal lines used in conventional interconnects is scaled down and their structure is brought closer in proximity, their parasitic resistance and capacitance increases. For example, it is well known that even in a copper-based low resistance metal process, it is critical to reduce interconnect capacitance to obtain speed benefits over an aluminum-based metal process.
Optical interconnects on the other hand, do not have parasitic issues and, therefore, are very appealing, particularly as integrated circuit size gets smaller. In an optical interconnect, speed is determined largely by the velocity of light in the interconnect medium and other associated complexities, such as modulation and de-modulation of signals from electrical to optical, and optical to electrical.
Photonic crystals have recently been of interest in optical technologies, due, in part, to their photonic band gaps. The term “photonic crystal” refers to a material and/or lattice of structures (e.g., an arrangement of pillars) with a periodic alteration in the index of refraction. A photonic crystal interacts with electromagnetic waves analogously to how a semiconductor crystal interacts with charge particles or their wave forms, i.e., photonic crystal structures are optical analogs of semiconductor crystal structures. The fundamental aspect of both photonic and semiconductor crystals is their periodicity. In a semiconductor, the periodic crystal structure of atoms in a lattice is one of the primary reasons for its observed properties. For example, the periodicity of the structure allows quantization of energy (E) levels and wave vector momentum (k) levels (band structure, E-k relationships). In a similar manner, photonic crystals have structures that allow the tailoring of unique properties for electromagnetic wave propagation. Similar to band gap energy in semiconductors, where carrier energies are forbidden, photonic crystals can provide a photonic band gap for electromagnetic waves, where the presence of particular wavelengths is forbidden. See Biswas, R. et al., Physical Review B, vol. 61, no. 7, pp. 4549-4553 (1999), the entirety of which is incorporated herein by reference.
Unlike semiconductors, photonic crystals are not limited to naturally occurring materials and can be synthesized easily. Therefore, photonic crystals can be made in a wide range of structures to accommodate the wide range of frequencies and wavelengths of electromagnetic radiation. Electromagnetic waves satisfy the simple relationship to the velocity of light:c=nλwhere c=velocity of light in the medium of interest, n=frequency and λ=wavelength. Radio waves are in the 1 millimeter (mm) range of wavelengths whereas extreme ultraviolet rays are in the 1 nanometer (nm) range. While band structure engineering in semiconductors is very complex, photonic band structure engineering in photonic crystals it is relatively simple. Photonic crystals can be engineered to have a photonic band structure that blocks predetermined wavelengths of light while allowing other wavelengths to pass through.
Photonic crystals can also demonstrate negative refraction. A material that shows a negative refractive index can refract electromagnetic waves with a flat surface. In contrast, conventional lenses have a positive refractive index, and therefore, have a curved surface. See Parimi, Patanjali V. et al., Nature, vol. 426, p. 404 (2003), the entirety of which is incorporated herein by reference, for a discussion of experimental results demonstrating negative refraction at microwave frequencies. See also Pendry, J. B., Physics Review Letters, vol. 85, no. 18, pp. 3966-3969 (2000), which is incorporated herein by reference.
It is desirable to have an interconnect for an integrated circuit for high speed performance even at smaller integrated circuit sizes. More particularly, it is desirable to have photonic crystal-based elements, including an optical interconnect and filters and lenses for optical interconnects for an integrated circuit.